It has long been known that thermoplastically processable styrene/acrylonitrile copolymers (SAN) may be imparted improved impact properties by the incorporation of rubbers. In ABS polymers, diene polymers are used as the rubbers for toughening, most notably at low temperatures; however, the relatively poor weathering and aging resistance make these materials less favored in more demanding applications. In some such applications, use has successfully been made of crosslinked acrylic acid ester polymers; these are the well known ASA copolymers. Such have been described in U.S. Pat. No. 3,055,859 and in German 1,260,135 and 1,911,882. Accordingly, the preferably crosslinked, rubbery acrylic acid ester polymer which serves as the grafting base (substrate) is first prepared by emulsion polymerization and the latex thus prepared is then grafted, preferably by emulsion, with a mixture of styrene and acrylonitrile. The art thus has long been recognized that improved impact strength, notched Izod, greater hardness and reduced shrinkage are associated with such ASA products which have as a grafting base a coarse polyacrylate latex having a mean particle diameter of about 150 to 800 nm and a narrow particle size distribution. Also, noted in the present context is the disclosure in U.S. Pat. No. 4,224,419 which disclosed an ASA based composition which contains two different graft copolymers of SAN onto crosslinked acrylates, and a hard copolymeric SAN component.
The art refers to "hard" and "soft" segments in terms of the relative position of their glass transition temperature vis a vis room temperature. "Hard" means T.sub.g above room temperature and "soft" means below room temperature, as well as to core/shell structures including structures which contain a multiplicity of shells. Multi-phase structured emulsion copolymers, including hard-soft and hard morphologies have been disclosed in EP 534,212 and in the documents referenced therein. Accordingly, graft copolymers with a hard core of polystyrene, a first butyl acrylate shell and an outer SAN shell have been disclosed in the art. Significantly, the '212 document disclosed a monomodal system containing particles having a styrene core and measuring less than 0.2 microns. Also presently relevant is a copending patent application, Ser. No. 08/955,857 which concerns a corresponding composition containing a monomodal size distribution of a grafted rubber phase.